Thank you! Very nice support as for open-source software! The issue is resolved now by the following steps:
ln -s /usr/local/ispconfig/server/mods-available/dns_module.inc.php /usr/local/ispconfig/server/mods-enabled/dns_module.inc.php
ln -s /usr/local/ispconfig/server/plugins-available/bind_plugin.inc.php /usr/local/ispconfig/server/plugins-enabled/bind_plugin.inc.php